A relatively recent development in the operation of access networks employing DSL connections is to move from fixed rate connections (where a user contractually agrees with the network operator/service provider a fixed rate (e.g. 2 Mbit/s) at which the DSL connection will operate and the network always attempts to set up a connection at exactly this speed) to rate adaptive connections where the DSL connection attempts to connect up at the fastest achievable (but reasonably stable) speed of connection (e.g. up to 8 Mbit/s).
Dynamic Line Management (DLM) is a technique for improving the stability of DSL connections. It is particularly useful when operating DSL connections at close to their maximum speed as occurs in rate adaptive connections because, under these conditions, external noise affecting the transmitted signal can cause the transceivers to be unable to successfully recover the signal to be transmitted with sufficient reliability to enable the connection to be maintained. If this occurs, the connection needs to be re-established. This operation is referred to as a re-synchronisation or a re-train and, during this operation, the end user will notice a loss of service. Re-synchronisations are particularly annoying to end users and it highly desirable to minimise their occurrence.
DLM seeks to minimise the rate of occurrence of re-synchronisations by automatically analysing DSL connections and varying certain parameters which can affect the likelihood of them re-occurring, for example by reducing the data rate or increasing the interleaving depth. Typically this is done by providing a number of different profiles each having various different sets of values for the parameters most likely to have an impact on the stability or otherwise of a DSL connection, and moving a particular connection between different profiles until a profile is found which has acceptable stability. For example, the profiles may specify a minimum and maximum data rate, a target (i.e. minimum) signal to noise margin (SNM) and a level of interleave (all of these parameters are discussed in greater detail below). In a typical rate adaptive profile, the minimum rate is set at some predetermined minimum acceptable speed (e.g. 512 Kbit/s) below which the line is no longer considered to be operating at a broadband speed connection and the maximum is set to the maximum rate which the equipment can support under ideal circumstances (i.e. where there is a very high signal to noise ratio for the received signals). The profiles are applied at the Digital Subscriber Line Access Multiplexer (DSLAM), which is usually housed at a local exchange, sometimes referred to as a central office in the United States, and which contains a number of DSL transceiver units as is well known in the art. In applying a given rate adaptive profile, the DSLAM will attempt to negotiate the greatest rate it can manage whilst still achieving the target SNM specified in the respective profile being applied to the connection—the aim is to find a profile such that the connection operates at the highest rate of data transfer that it can sustain without the line ever being forced into performing a resynchronisation—note that once a connection has synchronised it may respond to changes in the external noise affecting the line by performing “bit-swapping” (this involves changing the number of bits transmitted by each channel—i.e. if one channel finds that its measured signal to noise ratio has improved whilst another's has worsened, the former can start transmitting more bits while the latter transmits a correspondingly reduced amount) but it will keep the overall data transfer rate constant. In order to apply the profile the DSL transceivers measure the signal to noise ratio experienced during the synchronisation period on each channel and then determine how many bits (per symbol or per second etc.) can be transmitted in each channel based on this measurement.
The system would be fairly straightforward if the noise level was fairly constant as then the SNR measured at synchronisation would be representative of the SNR going forward. However, since the noise may comprise components of crosstalk, impulse noise and Repetitive Electrical Impulse Noise (REIN), all of which can vary with time the SNR measured at synchronisation may well not accurately reflect the SNR going forward in time. An incorrect noise estimation may result, in one direction (i.e. if the overall level of noise over all channels increases from the time that the synchronisation was performed by more than the amount catered for by the SNR target margin), in the DSLAM employing a rate which still results in instability or, in the other direction (i.e. if the overall noise reduces or does not ever increase as much as the SNR target margin catered for), one which is too conservative and so results in a poor data rate.
If the noise varies over relatively short periods, it is particularly difficult to estimate the noise and so SNR. Although the DSLAM may estimate the noise over a period which is adequate to observe such short term variation, the statistics of the noise in terms of peak to mean ratio and other measures can make it difficult to correctly compute the maximum available data rate that can be achieved. If the noise varies over longer time scales, it may be that the estimate used when the DSLAM established a connection, although correct at the time, is incorrect a short time later causing the connection to fail and require re-synchronisation. This can happen repeatedly. It may even be that the DSLAM transceiver is itself poorly implemented and so incorrectly estimates the noise.
In view of this, it is desirable to provide an improved DLM algorithm.